Erythropoietic protoporphyria (EPP) is a genetic disease in which ferrochelatase, the enzyme that inserts iron into protoporphyrin, is defective. In EPP, protoporphyrin accumulate in erythrocytes, leaks into about 5% of patients. Since it has been demonstrated that the vast majority of the protoporphyrin found in plasma, skin and liver derives from the erythrocytes, we propose that gene therapy directed at the bone marrow could cure EPP. This would be especially beneficial for patients with severe photosensitivity or incipient liver disease resistant to pharmacological treatment. Using a mouse model of EPP (homozygous recessive mutation), we have recently demonstrated that transplantation of bone marrow from normal syngeneic Balb/C donors into irradiated EPP recipients completely cures the disease phenotype. We have also shown that retrovirus-mediated transfer of human ferrochelatase cDNA into peripheral blood BFU-E from human EPP patients corrects the specific protoporphyrin-mediated fluorescence. In addition, we are developing improved gene transfer methods for hematopoietic stem cells by means of Lentiviral vectors and in vivo selection protocols, which will be validated and optimized in the EPP gene therapy model. Based on these preliminary results, our Specific Aims are as follows: Specific Aim 1: To demonstrate that retroviral transfer of human ferrochelatase cDNA into hematopoietic stem cells of EPP donor mice followed by their transplantation into EPP recipient mice will cure or significantly ameliorate the disease phenotype. Specific Aim 2: To compare the efficiencies of Murine Leukemia Virus-verus HIV-1 Lentivirus-based vectors to transduce murine hematopoietic stem cells and correct the EPP phenotype by bone marrow transplantation. Specific Aim 3: To evaluate the efficacy of gene therapy protocols for EPP that combine minimal myeloablation prior to bone marrow transplantation, Multi-drug Resistance (MDR) or Dihydrofolate Reductase (DHFR) retroviral vectors and corresponding in vivo selection regimens.